The cellular events in the immune response will be studied in vitro systems: 1. We will examine the function and nature of the nonantigen specific T cell mediator of helper activity in the humoral immune response. 2. The triggering requirements for B cell responses to antigens of various types and for various immunoglobulin classes will be compared. 3. Cell fractionation, differential mitogen responsiveness and measurement of various T cell functions will be used to examine T cell heterogeneity. 4. The characteristics of the immune response to allogeneic and syngeneic tumor antigens will be studied.